The present invention relates to a ceramic green body with improved mechanical strength and a method of producing it, and more particularly to a ceramic green body reinforced by fibrous ceramic additives and a method of producing it.
Ceramic sintered bodies are generally produced by first forming green bodies of certain shapes from ceramic powder and then sintering them. Since ceramics are not easy to work once sintered, they should be formed into desired shapes before sintering. Known as methods of producing such ceramic green bodies are a die pressing method, an injection molding method, a slip casting method in which ceramic slips are cast into gypsum molds, a cold isostatic pressing method by rubber presses, etc. In order to produce green bodies with extremely small thickness or complicated shapes, the slip casting method, the injection molding method, a molding method by a doctor blade, etc. are preferable.
In particular, the slip casting method is preferable because it can produce green bodies of thin and complicated shapes with extreme easiness. It may be conducted by dispersing ceramic starting material powder in water and casting the resulting slip into a gypsum mold, thereby causing the gypsum mold to absorb and remove water to provide a green body. Particularly when silicon nitride ceramic green bodies are produced, silicon nitride powder is mixed with sintering aides such as Y.sub.2 O.sub.3, etc. and other additive powder, and then the above slip casting method is conducted.
However, when a ceramic green body is produced by such a slip casting method, the force bonding ceramic powder particles is not necessarily large even though a binder, etc. is added to the ceramic slip, and so the resulting green body is likely to be cracked or broken because of poor mechanical strength. Further, because of dewatering and drying in the gypsum mold, the ceramic green body is shrinked, thus susceptible to cracking. Thus, it has been difficult to produce thin green bodies or those having complicated shapes without danger of breakage or cracking.
Incidentally, composite ceramics containing ceramic whiskers such as SiC whiskers are known. The ceramic whiskers serve as reinforcing fillers in the sintered ceramics. However, in the green bodies, the ceramic whiskers do not show sufficient reinforcing effects. Particularly when the slip casting method is employed, the deformation, cracking and even breakage of the green bodies are not effectively prevented by the addition of ceramic whiskers.
On the other hand, the addition of fibrous zirconia in the form of tow, yarn, woven fabrics, felts, roving, knits braids, paper and the like to cement composites is known (U.S. Pat. No. 3,736,160). However, in the cement composites, fibrous zirconia is not uniformly dispersed in a matrix, and rather it is impregnated with ceramic compounds. Therefore, it is difficult to produce green bodies of extremely thin or complicated shapes with fibrous zirconia in the form of tow, yarn, etc. impregnated with ceramic compounds.